Implantable cardiac pacemakers are a class of cardiac rhythm management devices that provide electrical stimulation in the form of pacing pulses to selected chambers of the heart. (As the term is used herein, a pacemaker is any cardiac rhythm management device with a pacing functionality regardless of any additional functions it may perform such as cardioversion/defibrillation.) Most pacemakers are used in the treatment of bradycardia by enforcing a minimum heart rate and/or restoring atrio-ventricular conduction in order to make up for a heart's inability to pace itself at an appropriate rhythm. Also included within the concept of cardiac rhythm is the manner and degree to which the heart chambers contract during a cardiac cycle to result in the efficient pumping of blood. Pacemakers have been developed which provide electrical pacing stimulation to one or both of the atria and/or ventricles at single or multiple sites during a cardiac cycle in an attempt to improve the coordination of atrial and/or ventricular contractions, termed cardiac resynchronization therapy. Such multiple paces are usually delivered to a heart chamber during a cardiac cycle with a pacing mode similar to that used for bradycardia pacing.
In pacemakers with sensing channels for sensing one or more heart chambers, the ventricular and/or atrial sensing channels are rendered refractory following certain events, such that certain sensed signals are ignored for the duration of the period. Sensing channels are rendered refractory in order to prevent the misinterpretation of electrogram signals resulting from a pace or a previous depolarization in the heart chamber sensed by the channel. A sensing channel may also be rendered refractory for a period of time in response to a pace or depolarization in a heart chamber different from the chamber sensed by the channel. Such refractory periods are called cross-chamber refractory periods.